Note: Descriptions are shown in the official language in which they were submitted.
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Description
APPARATUS AND METHOD FOR ADAPTIVE CHANNEL
QUALITY FEEDBACK IN A MULTICARRIER WIRELESS
NETWORK
Technical Field
[1] The present application relates generally to wireless communications and,
more
specifically, to a mechanism for providing channel quality feedback in a
multicarrier
wireless network.
Background Art
[21 Orthogonal frequency division multiplexing (OFDM) is a multicarrier
transmission
technique in which a user transmits on many orthogonal frequencies (or
subcarriers or
tones). The orthogonal subcarriers (or tones) are individually modulated and
separated
in frequency such that they do not interfere with one another. This provides
high
spectral efficiency and resistance to multipath effects. An orthogonal
frequency
division multiple access (OFDMA) system allows some subcarriers to be assigned
to
different users, rather than to a single user.
[31 The performance of a wireless network may be improved by implementing
channel
quality feedback. Receiving stations (e.g., subscriber stations) in a wireless
network
measure selected parameters of the received signal. The measured parameters
and,
optionally, calculated values derived from the measured parameters are then
transmitted back to the wireless network in a standard message, sometimes
called a
channel quality indicator (CQI) message. The network then uses the CQI
information
to optimize the signal transmitted in the forward channel (or downlink),
thereby
improving reception in the subscriber stations. Similar techniques may be used
by the
subscriber stations to improve performance in the reverse channel (or uplink).
[41 A variety of channel quality feedback techniques are commonly used in mul-
tichannel wireless networks, such as OFDM and OFDMA networks. However, mul-
tichannel wireless networks typically allocate a subband containing a group of
subcarriers (or tones) to each subscriber station and these conventional
channel quality
feedback techniques commonly transmit an absolute channel quality indicator
(CQI)
value for each subband. Transmitting an absolute CQI value for each subband
requires
a prohibitive amount of feedback overhead.
[51 Therefore, there is a need for improved OFDM and OFDMA transmission system
that minimize the amount of bandwidth required to provide channel quality
feedback.
[61
Disclosure of Invention
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Technical Problem
[7] The present invention provides method and apparatus for
transmitting/receiving channel quality information in a wireless communication
system using multicarrier.
[8] The present invention also provides method and apparatus for
transmitting/receiving channel quality information with reducing a feedback
overhead in a wireless communication system using multicarrier.
Technical Solution
[9] A transmitter is provided for use in a wireless network capable of
communicating according to a multi-carrier protocol, such as OFDM or OFDMA,
the transmitter capable of determining a total average signal level across N
subbands, each of the N subbands including a plurality of subcarriers, and
determining a first average signal level within a first one of the N subbands.
In
one embodiment, the transmitter comprises controller for generating a channel
quality indicator (CQI) feedback message comprising a first data field
indicating
the total average signal level and a second data field indicating the first
average
signal level; transmission module for transmitting the CQI feedback message to
the wireless network. In another embodiment, a base station is provided for
use
in a wireless network capable of communicating with a plurality of subscriber
stations according to a multi-carrier protocol. The base station transmits in
a
downlink to the plurality of subscriber stations using N subbands, where each
of
the N subbands comprises a plurality of subcarriers. The base station is
capable
of receiving from a first one of the plurality of subscriber stations a
channel
quality indicator (CQI) feedback message. The CQI feedback message comprises
a first data field indicating a total average signal level determined by the
first
subscriber station across the N subbands and a second data field indicating a
first
average signal level determined by the first subscriber station within a first
one of
the N subbands. The base station uses the CQI feedback message to allocate
selected ones of the N subbands for transmitting in the downlink to the first
subscriber station.
According to an aspect of the present invention, there is provided a
subscriber station for transmitting a feedback message in a wireless network
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capable of communicating according to a multi-carrier protocol, the subscriber
station comprising:
a calculator for determining a first Channel Quality Indicator (CQI) value
across N subbands, and determining a second CQI value within at least one
subband of the N subbands;
a controller for generating a feedback message comprising a first data
field indicating the first CQI value and a second data field representing a
value
indicating a relative difference between the second channel quality indicator
value
and the first channel quality indicator value; and
a transmission module for transmitting the feedback message to the
wireless network.
According to another aspect of the present invention, there is provided a
subscriber station for transmitting channel quality information in a wireless
network capable of communicating according to a multi-carrier protocol, the
subscriber station comprising:
a controller for estimating a channel type using a signal received from the
wireless network and generating a first feedback message including the channel
quality information having different format according to the estimated channel
type and for generating a second feedback message comprising a first data
field
indicating a first channel quality indicator value across N subbands and a
second
data field representing a value indicating a relative difference between a
second
channel quality indicator value within at least one subband of the N subbands
and
the first channel quality indicator value; and
a transmission module for transmitting the first and second feedback
messages to the wireless network.
According to a further aspect of the present invention, there is provided a
transmission method for transmitting a feedback message by a subscriber
station
in a wireless network capable of communicating according to a multi-carrier
protocol, the transmission method comprising:
determining a first Channel Quality Indicator (CQI) value across N
subbands, and determining a second CQI value within at least one one subband
of
the N subbands;
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generating a feedback message comprising a first data field indicating the
first CQI value and a second data field representing a value indicating a
relative
difference between the second channel quality indicator value and the first
channel quality indicator value; and
transmitting the feedback message to the wireless network.
According to a further aspect of the present invention, there is provided a
transmission method for transmitting channel quality information in a wireless
network capable of communicating according to a multi-carrier protocol, the
transmission method comprising:
estimating a channel type using a signal received from the wireless
network and generating a first feedback message including the channel quality
information having different format according to the estimated channel type;
generating a second feedback message comprising a first data field
indicating a first channel quality indicator value across N subbands and a
second
data field representing a value indicating a relative difference between a
second
channel quality indicator value within at least one subband of the N subbands
and
the first channel quality indicator value; and
transmitting the first and second feedback messages to the wireless
network.
According to a further aspect of the present invention, there is provided a
base station for receiving a feedback message in a wireless network capable of
communicating according to a multi-carrier protocol, the base station
comprising:
a transmitter for transmitting a signal in a downlink to a plurality of
subscriber stations using N subbands; and
a receiver for receiving from a first one of the plurality of subscriber
stations a feedback message,
wherein the feedback message comprises a first data field indicating a first
Channel Quality Indicator (CQI) value determined across the N subbands and a
second data field representing a value indicating a relative difference
between a
second CQI value determined within at least one subband of the N subbands and
the first channel quality indicator value.
According to a further aspect of the present invention, there is provided a
method for receiving a feedback message by a base station in a base station
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capable of communicating with a plurality of subscriber stations according to
a
multi-carrier protocol, the method comprising:
transmitting a signal in a downlink to a plurality of subscriber stations
using N subbands; and
receiving from a first one of the plurality of subscriber stations a feedback
message,
wherein the feedback message comprises a first data field indicating a first
Channel Quality Indicator (CQI) value determined across the N subbands, and a
second data field representing a value indicating a relative difference
between a
second CQI value determined within at least one subband of the N subbands and
the first channel quality indicator value.
In some embodiments, the controller determines the total average signal
level across the N subbands based on a subset of all of the subcarriers in the
N
subbands.
In some embodiments, the third data field indicates the second average
signal level relative to the total average signal level.
In some embodiments, the method further comprises determining a second
average signal level within a second one of the N subbands, and wherein the
CQI
feedback message further comprises a third data field indicting the second
average signal level.
In some embodiments, the third data field indicates the second average
signal level relative to the total average signal level.
In some embodiments, the CQI feedback message further comprises a
third data field indicating a second average signal level determined by the
first
subscriber station within a second one of the N subbands.
In some embodiments, the third data field indicates the second average
signal level relative to the total average signal level.
In some embodiments, the method further comprises identifying in the
CQI feedback message a third data field indicating a second average signal
level
determined by the first subscriber station within a second one of the N
subbands.
In some embodiments, the first and second average signal level is
respectively quantized by different step size each other.
Advantageous Effects
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[10] The present invention reduces a feedback overhead in a wireless
communication system using multicarrier, so that channel quality information
can
be efficiently transmitted/received.
Brief Description of the Drawings
[11] For a more complete understanding of the present disclosure and its
advantages, reference is now made to the following description taken in
conjunction with the accompanying drawings, in which like reference numerals
represent like parts:
[12] FIGURE 1 illustrates an exemplary wireless network that implements
adaptive channel quality feedback techniques in an OFDM network according to
the principles
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WO 2007/015627 PCT/KR2006/003034
of the present disclosure;
[131 FIGURE 2a is a high level block diagram of a conventional OFDMA
transmitter
according to one embodiment of the disclosure;
[141 FIGURE 2b is a high level block diagram of a conventional OFDMA receiver
according to one embodiment of the disclosure;
[151 FIGURE 3 illustrates resource allocation according to the principles of
the present
disclosure;
[161 FIGURE 4 is a flow diagram illustrating the feedback of CQI information
according
to the principles of the disclosure;
[171 FIGURE 5 illustrates CQI feedback according to one embodiment of the
present
disclosure;
[181 FIGURE 6 illustrates CQI feedback according to an alternate embodiment of
the
present disclosure;
[191 FIGURE 7 illustrates multiple CQI feedback formats according to the
principles of
the present disclosure; and
[201 FIGURE 8 illustrates CQI feedback rate selection based on the operating
carrier
frequency.
Best Mode for Carrying Out the Invention
[211 Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below,
it may be advantageous to set forth definitions of certain words and phrases
used
throughout this patent document: the terms conclude and comprise, as well as
derivatives thereof, mean inclusion without limitation; the term or, is
inclusive,
meaning and/or; the phrases associated with and associated therewith, as well
as
derivatives thereof, may mean to include, be included within, interconnect
with,
contain, be contained within, connect to or with, couple to or with, be
communicable
with, cooperate with, interleave, juxtapose, be proximate to, be bound to or
with, have,
have a property of, or the like; and the term controller means any device,
system or
part thereof that controls at least one operation, such a device may be
implemented in
hardware, firmware or software, or some combination of at least two of the
same. It
should be noted that the functionality associated with any particular
controller may be
centralized or distributed, whether locally or remotely. Definitions for
certain words
and phrases are provided throughout this patent document, those of ordinary
skill in the
art should understand that in many, if not most instances, such definitions
apply to
prior, as well as future uses of such defined words and phrases.
[221 FIGURES 1 through 6, discussed below, and the various embodiments used to
describe the principles of the present disclosure in this patent document are
by way of
illustration only and should not be construed in any way to limit the scope of
the
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disclosure. Those skilled in the art will understand that the principles of
the present disclosure
may be implemented in any suitably arranged communication system.
[23] A channel quality indicator (CQI) feedback technique is disclosed for use
in
multicarrier networks. A receiving device (e.g., a subscriber station)
determines the average
channel quality over the entire frequency range used by the network. The
relative difference
between the average channel quality over the entire frequency range and the
subband average
channel quality is sent back to the transmitting device (for example, Base
station). Relative
channel quality indication (RCQI) levels used for reducing feedback overhead
when
transmitting channel quality information are defined and an RCQI level is fed
back for each
subband of the wireless network.
[24] FIGURE 1 illustrates exemplary wireless network 100, which implements
adaptive
channel quality feedback techniques in an exemplary OFDMA (or OFDM) network
according
to the principles of the present disclosure. In the illustrated embodiment,
wireless network 100
includes base station (BS) 101, base station (BS) 102, base station (BS) 103,
and other similar
base stations (not shown). Base station 101 is in communication with base
station 102 and base
station 103. Base station 101 is also in communication with Internet 130 or a
similar IP-based
network (not shown).
[25] Base station 102 provides wireless broadband access (via base station
101) to
Internet 130 to a first plurality of subscriber stations within coverage area
120 of base station
102. The first plurality of subscriber stations includes subscriber station
111, which may be
located in a small business (SB), subscriber station 112, which may be located
in an enterprise
(E), subscriber station 113, which may be located in a WiFi hotspot (HS),
subscriber station
114, which may be located in a first residence (R), subscriber station 115,
which may be located
in a second residence (R), and subscriber station 116, which may be a mobile
device (M), such
as a cell phone, a wireless laptop, a wireless PDA, or the like.
[26] Base station 103 provides wireless broadband access (via base station
101) to
Internet 130 to a second plurality of subscriber stations within coverage area
125 of base station
103. The second plurality of subscriber stations includes subscriber station
115 and subscriber
station 116. In an exemplary embodiment, base stations 101-103 may communicate
with each
other and with subscriber stations 111-116 using OFDM or OFDMA techniques.
[27] Base station 101 may be in communication with either a greater number or
a lesser
number of base stations. Furthermore, while only six subscriber stations are
depicted in
FIGURE 1, for instance, it is understood that wireless network 100 may provide
wireless
broadband access to additional subscriber stations. It is noted that
subscriber station 115 and
subscriber station 116 are located on the edges of both coverage area 120 and
coverage area
125. Subscriber station 115 and subscriber station 116 each
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communicate with both base station 102 and base station 103 and may be said to
be operating
in handoff mode, as known to those of skill in the art.
[28] Subscriber stations 111-116 may access voice, data, video, video
conferencing, and/
or other broadband services via Internet 130. In an exemplary embodiment, one
or more of
subscriber stations 111-116 may be associated with an access point (AP) (not
shown in
FIGURES) of a WiFi WLAN. Subscriber station 116 may be any of a number of
mobile
devices, including a wireless-enabled laptop computer, personal data
assistant, notebook,
handheld device, or other wireless-enabled device. Subscriber stations 114 and
115 may be,
for example, a wireless-enabled personal computer (PC), a laptop computer, a
gateway, or
another device.
[29] Hereinafter, the configuration of the transmitter and the receiver
according to the
embodiment of the present invention will be described referring to FIGURES 2a
and 2b.
[30] FIGURE 2a according to the embodiment of the present invention is a high-
level
diagram of orthogonal frequency division multiple access (OFDMA) transmitter
200.
FIGURE 2b according to the embodiment of the present invention is a high-level
diagram of
orthogonal frequency division multiple access (OFDMA) receiver 250. OFDMA
transmitter
200 or OFDMA receiver 250, or both, may be implemented in any of base stations
101-103
of wireless network 100. Similarly, OFDMA transmitter 200 or OFDMA receiver
250, or
both, may be implemented in any of subscriber stations 111-116 of wireless
network 100.
[31] OFDMA transmitter 200 comprises quadrature amplitude modulation (QAM)
modulator 205, serial-to-parallel (S-to-P) block 210, Size N Inverse Fast
Fourier Transform
(IFFT) block 215, parallel-to-serial (P-to-S) block 220, add cyclic prefix
block 225, and up-
converter (UC) 230. OFDMA receiver 250 comprises down- converter (DC) 255,
remove
cyclic prefix block 260, serial-to-parallel (S-to-P) block 265, Size N Fast
Fourier Transform
(FFT) block 270, parallel-to-serial (P-to-S) block 275, quadrature amplitude
modulation
(QAM) demodulator 280, and channel quality indicator (CQI) calculation block
285.
[32] At least some of the components in FIGURES 2a and 2b may be implemented
in
software while other components may be implemented by configurable hardware or
a
mixture of software and configurable hardware. In particular, it is noted that
the FFT blocks
and the IFFT blocks described in this disclosure document may be implemented
as
configurable software algorithms, where the value of Size N may be modified
according to
the implementation.
[33] Furthermore, although the present disclosure is directed to an embodiment
that
implements the Fast Fourier Transform and the Inverse Fast Fourier Transform,
this is by
way of illustration only and should not be construed so as to limit the scope
of this
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WO 2007/015627 PCT/KR2006/003034
disclosure. It will be appreciated that in an alternate embodiment of the
disclosure, the
Fast Fourier Transform functions and the Inverse Fast Fourier Transform
functions
may easily be replaced by Discrete Fourier Transform (DFT) functions and
Inverse
Discrete Fourier Transform (IDFT) functions, respectively. It will be
appreciated that
for DFT and IDFT functions, the value of N may be any integer number (i.e., 1,
2, 3, 4,
etc.), while for FFT and IFFT functions, the value of N may be any integer
number that
is a power of two (i.e., 1, 2, 4, 8, 16, etc.).
[341 In OFDMA transmitter 200, QAM modulator 205 receives a set of information
bits
and modulates the input bits to produce a sequence of frequency-domain
modulation
symbols, wherein the modulation method is selected according to the
predetermined
modulation method such as QPSK, QAM, 16 QAM, 64 QAM. In selected feedback
control message, these information bits may include channel quality indicator
(CQI)
information, as described herein. Thus the controller can be connected to the
preamble
of the QAM modulator (205), which generates CQI feedback message according to
the
present invention to be described below from the channel quality information.
Serial-
to-parallel block 210 converts (i.e., de-multiplexes) the serial QAM symbols
to parallel
data to produce N parallel symbol streams where N is the IFFT/FFT size used in
transmitter 200 and receiver 250. Size N IFFT block 215 then performs an IFFT
operation on the N parallel symbol streams to produce time-domain output
signals.
Parallel-to-serial block 220 converts (i.e., multiplexes) the parallel time-
domain output
symbols from Size N IFFT block 215 to produce a serial time-domain signal. Add
cyclic prefix block 225 then inserts a cyclic prefix to the time-domain
signal.
[351 Finally, up-converter 230 modulates (i.e., up-converts) the output of add
cyclic
prefix block 225 to RF frequency for transmission via the forward channel or
reverse
channel, depending on whether OFDMA transmitter 200 is implemented in a base
station or a subscriber station. The signal from add cyclic prefix block 225
may also be
filtered at baseband before conversion to RF frequency. The time-domain signal
transmitted by OFDMA transmitter 200 comprises multiple overlapping sinusoidal
signals corresponding to the data symbols transmitted.
[361 In OFDMA receiver 250, an incoming RF signal is received from the forward
channel or reverse channel, depending on whether OFDMA receiver 250 is im-
plemented in a base station or a subscriber station. OFDMA receiver 250
reverses the
operations performed in OFDMA transmitter 200. Down-converter 255 down-
converts
the received signal to baseband frequency and remove cyclic prefix 260 removes
the
cyclic prefix to produce the serial time-domain baseband signal. Serial-to-
parallel
block 265 converts the time-domain baseband signal to parallel time domain
signals.
Size N FFT block 270 then performs an FFT algorithm to produce N parallel
frequency-domain signals. Parallel-to-serial block 275 converts the parallel
frequency-
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domain signals to a sequence of QAM data symbols. QAM demodulator 280 then
demodulates the QAM symbols to recover the original input data stream.
[37] In the exemplary embodiment in FIGURE 2b, channel quality indicator (CQI)
calculation block 285 measures the signal at the output of Size N FFT block
270 to
determine one or more channel quality parameters according to the selected
channel quality
feedback algorithm according to the present invention, for example, by
receiving a CQI
feedback message from a subscriber station. However, in alternate embodiments,
channel
quality indicator (CQI) calculation block 285 may measure the received signal
at other
points in the receive path, such as at the output of remove cyclic prefix
block 260.
[38] According to the principles of the present disclosure, CQI calculation
block 285 is
capable of determining the average channel quality across all subcarriers at
the output of
Size N FFT block 270 (i.e., the entire frequency range of carrier used by
network 100).
CQI calculation block 285 also determines the relative difference between the
average
channel quality and the subband average channel quality in each subband. This
CQI
information (as shown in FIGURE 2a) is then sent back to the transmitting
device using
relative channel quality indication (RCQI) levels as described herein. In
addition, in order
to assign the selected subband among the N subband for transmitting a down
link to the
subscriber station, the present invention is equipped with the controller
connected to the
CQI calculator, such that the CQI feedback message of the present invention
can be used.
[39] FIGURE 3 illustrates the allocation of subcarriers in a frequency domain
scheduling scheme in OFDMA wireless network 100 according to the principles of
the
present disclosure. In this example, a total of 512 OFDM subcarriers (or
tones) are divided
into 8 groups (or subbands) of 64 contiguous subcarriers (SCs) each. By way of
example,
the first subband, SBI, contains subcarriers SC 1-SC64, the second subband,
SB2, contains
subcarriers SC65-SC128, and so forth. The eighth (last) subband, SB 8,
contains
subcarriers SC449-SC512.
[40] A given subscriber station (e.g., SS 116 or SS 115) may be allocated one
or more
of these subbands. In FIGURE 3, the eight subbands, SB1-SB8, are allocated
according to
channel fading at the receiver for the case of two subscriber stations, SS 115
and SS 116.
The received signals at SS 116 and SS 115 experience frequency-selective
fading due to
multipath effects. Curve 330a represents a flat-fading characteristic. Dotted-
line curve
31 Oa represents the frequency selective fading of the downlink signal from BS
102 seen by
the receiver of SS 116. Solid-line curve 320a represents the frequency
selective fading of
the downlink signal from BS 102 seen by the receiver of SS 115.
[41] In accordance with the principles of the present disclosure, SS 115 and
SS 116
measure one or more parameters of the received downlink signals and report
channel
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quality indicator (CQI) data back to BS 102. BS 102 uses this channel quality
feedback
information to schedule SS 115 and SS 116 to receive in certain subbands. In
the example in
FIGURE 3, SS 116 is schedule on subbands SB1, SB2, SB6, SB7 and SB8, where the
channel
quality for SS 116 is better than the channel quality for SS 115. SS 115 is
scheduled on
subbands SB3, SB4 and SB5, where SS 115 has relatively higher received power.
Thus, the
relative fading at each subscriber station is used to determine subband
allocation 350 near the
bottom of FIGURE 3. By scheduling subscriber stations on subbands with
relatively higher
channel quality, the SINR of the subscriber stations and the overall system
capacity may be
greatly improved.
[42] FIGURE 4 depicts flow diagram 400, which illustrates the feedback of CQI
information according to the principles of the disclosure. Initially, CQI
calculation block 285
(in SS 115, for example) calculates a total average channel quality indicator
(CQI) value for the
entire bandwidth occupied by the 512 subcarriers in FIGURE 3 (process step
410). Next, CQI
calculation block 285 calculates an average CQI for each of the subbands SB 1-
SB8 (process
step 420). CQI calculation block 285 then determines the relative difference
between the total
average CQI value and each individual subband average CQI value (process step
430). Next,
CQI calculation block 285 (or an equivalent functional block in SS 115)
constructs a CQI
message based on the total average CQI and relative subband CQI (RSCQI)
(process step 440).
Finally, SS 115 transmits the CQI message to BS 102 (process step 450).
[43] It should be noted that it is not strictly necessary to calculate the
total average CQI
value based on the entire bandwidth occupied by all of the subcarriers. In
alternate
embodiments, the total average CQI value may be based on a representative
subset of the
subcarriers. For example, the total average CQI value may be determined from
the subcarriers
in subbands SB2 through SB7, while subcarriers in subbands SB1 and SB8 are not
used to
determine the total average CQI value. In another example, the total average
CQI value may be
calculated using only the even numbered subcarriers (or only the odd numbered
subcarriers)
from all of the subbands, or from less than all of the subbands. These
alternative methods
require less processing power, but may provide less accurate estimates for the
total average CQI
value.
[44] Similarly, it is not strictly necessary to calculate the average CQI
value within a
particular subband using all of the subcarriers in the subband. In alternate
embodiments, the
average CQI value in a subband may be based on a representative subset of the
subcarriers in
that subband. For example, the relative CQI value for subband SB 1 may be
determined from
only the odd subcarriers, or only the even subcarriers, in subband SB1. In
another example, the
relative CQI value for subband SB I may be determined from, for example, 32
randomly
selected ones of the 64 subcarriers in subband SB 1.
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[451 FIGURE 5 illustrates CQI feedback according to one embodiment of the
present
disclosure. In FIGURE 5, the received signal level for SS 115 is shown across
the
entire 512 subcarriers of SB1 through SB8. Three relative signal levels are
shown by
horizontal solid lines. The middle line represents the total average signal
level, namely
x dB, across the entire spectrum. The bottom line represents a power level
that is 3 dB
below the total average, namely (x-3) dB. The top line represents a power
level that is
3 dB above the total average, namely (x+3) dB. The (x-3) dB line, the x dB
line, and
the (x+3) dB line define four regions that represent relative channel quality
indication
(RCQI) levels.
[461 The first RCQI level (RCQI = 0) is the region below the (x-3) dB line.
The second
RCQI level (RCQI = 1) is the region between the (x-3) dB line and the x dB
line. The
third RCQI level (RCQI = 2) is the region between the (x+3) dB line and the x
dB line.
The fourth RCQI level (RCQI = 3) is the region above the (x+3) dB line.
[471 In the example of FIGURE 5, dotted lines 501-508 represent the average
CQI level
within each of the subbands SB1-SB8 respectively. The location of each of
dotted lines
501-508 within the four regions defined by the RCQI levels 0, 1, 2, and 3
determines
the RCQI value for the corresponding subband. In FIGURE 5, RCQI values of 0,
1, 3,
3, 2, 1, 0, and 0 are respectively fed back to BS 102 as the channel quality
indicators
for subbands SB 1-SB8, respectively.
[481 TABLE 1 illustrates an example of selected portions of a CQI feedback
message
according to the principles of the present disclosure.
[491
QUANTITY NUMBER OF BITS
Total Average CQI BO
SB1 CQI B1
SB2 CQI B2
SB3 CQI B3
SBn CQI Bn
TABLE 1
[501 The CQI message uses BO bits to quantize and indicate the average CQI
across the
whole bandwidth. The CQI message uses B 1, B2, Bn bits, respectively, to
indicate the
average CQI for subbands SB 1 through SBn, respectively. Thus, the CQI
feedback
overhead is the sum (BO + B 1 + + Bn) bits.
[511 An exemplary CQI feedback message for FIGURE 5 is shown in TABLE 2.
[521
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Avg. SB1 SB2 SB3 SB4 SB5 SB6 SB7 SB8
1011 00 01 11 11 10 01 00 00
TABLE 2
[531 The total average CQI across the entire spectrum is indicated by four (4)
bits
(B0=4) that may represent up to 16 different levels of CQI. These 16 levels
may, for
example, indicate CQI in increments of 1 dB for the range from 0.0 dB to +15
dB. The
relative CQI for each of subbands SB1-SB8 is indicated using two (2) bits.
With 2-bits,
4 levels of RCQI may be indicated. In this example, the total average CQI is
11 dB
(1011).
[541 In an alternate embodiment of the disclosure, an effective signal-to-
noise (SNR) is
calculated and fed back to the transmitter instead of the average CQI. The
effective
SINR may be calculated based on the channel capacity formula. First, the
average
channel capacity is calculated using Shannon capacity formula:
[551
K
C = 1 = Y log(1 + SNRk)
K k=1
[561 where K is the total number of subcarriers used for effective SNR
calculation and k
is the subcarrier index. The effective SNR may then be calculated as below:
[571
SNREFF = ZC-1
[581 FIGURE 6 illustrates CQI feedback according to an alternate embodiment of
the
present disclosure. In FIGUER 6, unequal quantization levels are used to
quantize the
subband RCQI levels. The total average signal level across the whole band is
assumed
to be 0 dB for convenience. The average signal levels within subbands SB 1-SB8
are
indicated by dotted lines 601-608, respectively. In general, a subscriber
station is
scheduled on subbands at higher CQI to maximize the received signal-to-
interference
and noise ratio (SINR). Therefore, it is important to characterize the updates
relative to
the average in an accurate manner. A subscriber station is less likely to be
scheduled
on a subband that is in a down-fade. Therefore, a down-fade may be quantized
with a
lower granularity.
[591 In the example of FIGURE 6, each up-fade relative to the 0 dB average is
char-
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acterized by one of three RCQI levels: 1, 2 and 3. An RCQI of 1 indicates the
region
between 0 dB and 2 dB. An RCQI of 2 indicates the region between 2 dB and 4
dB. An
RCQI of 3 indicates the region above 4 dB. However, a down fade is
characterized by
only a single RCQI level: 0. The four RCQI levels may be indicated by two
binary bits,
resulting in a 16-bit overhead for the eight (8) subband case. In addition, 4
bits may be
used to quantize the total average CQI across the whole bandwidth. Therefore,
the total
overhead in this case is 20 bits. In the example of FIGURE 6, only three
subbands,
namely SB3, SB4 and SB5 are above the average CQI. These three subbands, SB3,
SB4 and SB5, are indicated by RCQI levels of 2, 3 and 1 respectively. All
other
subbands are below 0 dB and are denoted by RCQI level 0.
[601 Additionally, there is no requirement that the step size of quantization
levels be
equal. For example, in FIGUER 6, the step size for RCQI levels 1 and 2 are
both 2 dB.
However, in an alternate embodiment, an RCQI of 1 may indicate the region
between 0
dB and 2 dB, while an RCQI of 2 may indicate the region between 2 dB and 6 dB.
Thus, the step size is 2 dB for RCQI level 1 and 4 dB for RCQI level 2.
[611 In another embodiment of the disclosure, the relative CQI values for the
subbands
are calculated relative to the average CQI in time and frequency. This average
CQI
value then represents the long-term CQI value due to path loss and shadow
fading. By
averaging over time and frequency, the effect of Doppler due to fading is
averaged out.
In such an embodiment, the total average CQI may be fed back at a relatively
low rate.
This is due to the fact that channel gain due to path loss and shadow fading
varies very
slowly as a function of time. However, the instantaneous subband CQI can be
calculated relative to the long-term average CQI and fed back more frequently.
[621 In one embodiment of the present disclosure, different CQI feedback
formats may
be selected based on the channel type (or channel characteristics). The
channel type
may be estimated using the reference preamble or pilot signal transmitted from
BS
102. The channel type includes, among other things, the extent of multipath or
frequency-selectivity in the channel. In case of a single-path or flat-fading
channel,
only a single RCQI value is fed back at a given time because of the absence of
frequency-selectivity in the channel. Therefore, the CQI format contains a
single RCQI
value which applies to all the subbands in the frequency domain. On the other
hand, in
a multi-path frequency-selective channel, different subbands see different
fading.
Therefore, the RCQI format may include subband RCQI values as described above.
After estimating the channel type, the CQI format is determined and CQI value
are fed
back to the base station based on the selected CQI format.
[631 FIGURE 7 illustrates multiple CQI feedback formats according to the
principles of
the present disclosure. By way of example, in a flat-fading channel type,
Format A is
used to transmit a single CQI value to BS 102. In a slightly frequency-
selective fading
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channel type, Format B is used to transmit two RCQI values to BS 102. In a
moderately frequency-selective fading channel type, Format C is used to
transmit four
RCQI values to BS 102. Finally, in a highly frequency-selective fading channel
type,
Format D is used to transmit eight RCQI values to BS 102.
[641 In a wireless mobile system, a Doppler shift is observed due to the
relative mobility
between the transmitter and the receiver. In a cellular system, the base
stations are at
fixed locations. Therefore, the Doppler shift occurs due to the mobility of
the
subscriber station. The Doppler shift is function of the subscriber station
speed and the
carrier frequency and is written as D = fv/C, where C is the speed of light, f
is the
carrier frequency, and v is the subscriber station speed.
[651 The channel quality varies faster as a function of time for higher
Doppler relative to
lower Doppler. Similarly, the channel quality varies faster as function of
time at a
higher carrier frequency for the same subscriber station speed. In order for
to obtain
accurate channel estimates for scheduling, the CQI feedback rate may be higher
for a
higher carrier frequency. Thus, the CQI feedback rate may be selected
adaptively
based on the operating frequency.
[661 FIGURE 8 depicts flow diagram 800, which illustrates CQI feedback rate
selection
based on the operating carrier frequency according to the principles of the
present
disclosure. Wireless systems may be deployed under a variety of carrier
frequencies.
This affects the CQI feedback rate. By way of example, to obtain the same
performance, the CQI feedback rate may be four (4) times greater in a 3.6 GHz
wireless network than in a 900 MHz wireless network.
[671 In FIGURE 8, the carrier frequency of the wireless network is initially
determined
(process step 810). Next, it is determined whether the carrier frequency is
900 MHz
(process step 820). If yes, the basic feedback rate is set to R updates/second
for a 900
MHz system (process step 830). If no, the ratio, K, between the actual
operating
frequency, f, and the 900 MHz reference frequency is calculated (process step
840).
The CQI feedback rate is then selected as KR updates/second (process step
850).
[681 The present invention reduces a feedback overhead in a wireless
communication
system using multicarrier, so that channel quality information can be
efficiently
transmitted/received.
[691 Although the present disclosure has been described with an exemplary
embodiment,
various changes and modifications may be suggested to one skilled in the art.
It is
intended that the present disclosure encompass such changes and modifications
as fall
within the scope of the appended claims.
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